There has been a trend in recent years for a laser light source which is used as alight source in an optical pickup apparatus for reproducing information which has been recorded in an optical disc and for recording information on an optical disc, toward a shorter wavelength. For example, laser light sources with 390-420 nm wavelength, such as a blue-violet semiconductor laser, are reaching the stage of practical application. By using these blue-violet laser light sources, information of 15-20 GB can be recorded on an optical disc with a diameter of 12 cm by using an objective lens with the same numerical aperture (NA) as that for DVD (Digital Versatile Disc), and information of 23-25 GB can be recorded onto an optical disc with a diameter of 12 cm by using an objective lens with increased NA up to 0.85.
As an example of an optical disc using the above-described objective lens with NA 0.85, there is cited a BD (Blu-ray Disc). Since increased comma is generated because of a tilt (skew) of the disc, a BD has been designed so that a protective layer has thinner thickness (which is 0.1 mm, while that of DVD is 0.6 mm) than that of DVD, to reduce the amount of comma caused by the skew.
On the other hand, it is considered that an optical disc player/recorder (optical information recording reproducing apparatus) is worthless as a product when the optical disc player/recorder is capable of recording/reproducing information just for BDs properly. Taking account of a fact that, at present, DVDs and CDs (Compact Discs) storing various kinds of information have been on the market, it is not sufficient that the optical disc player/recorder can record/reproduce information just for BDs, and an attempt providing an optical disc player/recorder capable to record/reproduce information also for DVDs and CDs which have already been owned by users, leads to enhancement of a commercial value of the optical disc player/recorder for BDs. From such the background, an optical pickup apparatus installed in the high-density optical disc player/recorder is required to be capable of appropriately recording/reproducing information not only for BDs but also for a DVDs and a CDs.
As a method by which information can be adequately recorded/reproduced while the compatibility is maintained to anyone of BDs, DVDs and CDs, there can be considered a method to selectively switch an optical system for BDs and an optical system for DVDs and CDs, corresponding to a recording density of an optical disc on which information is recorded/reproduced. However, it is disadvantageous for the size-reduction and increases a cost, because plural of optical systems are needed.
Accordingly, in order to simplify a structure of the optical pickup apparatus and to intend a reduction of its cost, it is preferable to form the optical system for BDs and the optical system for DVDs and CDs into a common optical system, to reduce the number of optical parts forming the optical pickup apparatus as much as possible, even in the optical pickup apparatus with compatibility. Then, providing the common objective lens which is arranged with facing an optical disc, is most advantageous for the simplification of the structure and for cost reduction of the optical pickup apparatus. In order to obtain a common objective lens for plural kinds of optical discs which use different wavelengths for recording/reproducing information, it is required that a diffractive structure having a wavelength dependency for the spherical aberration, is formed in the objective optical system.
JP-A No. 2008-293630 has disclosed an objective lens which includes a structure in which two basic structures each provided as a diffractive structure are overlapped with each other and can be commonly used for three types of optical discs, and also has disclosed an optical pickup apparatus in which this objective optical system is mounted.
Two of the three types of optical disc disclosed in JP-A No. 2008-293630 are DVDs and CDs. With respect to the rest, JP-A No. 2008-293630 has just touched about BD, and all of its examples actually employ HD-DVD, which suggests that JP-A No. 2008-293630 has been provided with emphasizing a compatibility of three types of optical disc of DVD, CD and HD-DVD rather than BD. The inventor has studied an applicability of the examples of the application to achieving compatibility of three types of optical disc of BDs, DVDs, and CDs, and has found an issue that a working distance of the objective lens for CD becomes so small that the objective lens can touch an optical disc during recording and reproducing operations. Herein, a working distance means a distance along the optical axis from a surface of the optical disc to a surface vertex of the optical surface of the objective lens facing the optical discs.